This invention relates to the control and protection of a synchronous motor, and in particular it relates to the remote determination of slip frequency which is useful for the control and protection of a brushless salient pole synchronous motor during starting of the motor.
The starting of a three phase synchronous motor is a relatively critical operation. During the starting of a synchronous motor, field excitation is applied when the rotor is rotating at a speed near the synchronous speed to effect synchronism with the power source. The field excitation must be applied after the rotor has reached a certain minimum speed for the rotor and connected load to be pulled into synchronism. If the field excitation is applied too early, excessive current torque and vibration will occur. Also, field excitation must be applied before the rotor reaches a speed from which it will pull into step without excitation, and this is of particular importance for motors with comparatively light loads. If a motor should pull into step without field excitation, the motor may have synchronized on the wrong polarity, and if excitation is then applied large stator currents will occur with undesirable results. In addition, if a motor operates for an excessive time before excitation is applied, overheating will occur which could cause damage to the motor windings.
Every motor is designed with a particular "pull in" speed, that is with a particular rotor speed where application of field excitation will effect synchronism with the power source. This pull in speed is normally in the range of about 94 to 99% of synchronous speed. Thus each combination of motor and load has a particular pull in speed below which field excitation should not be applied, and it may have a slightly higher speed where it will pull into synchronism without excitation. The two speeds may be quite close together and a rapidly accelerating motor could reach pull in speed in perhaps 5 to 8 seconds and pass to the higher speed very quickly. It will be seen that the starting operation is quite critical and the field excitation is normally applied automatically (i.e., by machine) as it is difficult for an operator to respond in the very short time available and to initiate application of field excitation.
The criticality of starting a three phase synchronous motor has been recognized in the prior art, and it is common to have a tachometer mounted on or mechanically connected to the motor to effect synchronism either automatically or by an operator when the motor reaches a predetermined pull in speed. A mechanical arrangement for obtaining an indication of speed is subject to wear and may not be desirable. Also it is not always convenient to have a display of motor speed at the operator position. It is, of course, possible to obtain from the tachometer a signal representing motor speed and to conduct this signal to a remote location. This however requires wires or other signal carrying means.
In another prior art arrangement for starting synchronous motors, the induced current is measured in the field winding of the motor. This induced current fluctuates at slip frequency and the measurement can therefore be used to determine rotor speed. The determination of rotor speed thus requires the use of slip rings, which makes it an undesirable arrangement for use in hazardous atmospheres. The signal is obtained at the operating position, i.e., at the motor, and if it is to be used remotely, for example, at a remote switching location, then wires or other signal carrying means must run to the remote location. It should be noted that the signal is not available in brushless synchronous motors where the field windings are not available externally of the motor.
It is therefore an object of the invention to provide means for convenient remote detection of the slip frequency of a two, three or six-phase salient pole synchronous motor.
It is also an object of the invention to provide improved apparatus for controlling the starting of a three phase salient pole synchronous motor.
It is another object of the invention to provide apparatus for controlling the starting of a three phase salient pole synchronous motor, which apparatus may be located remotely from the motor, for example at the switchgear for use with the motor installation, and which requires no additional wires running from the motor.